Rare-earth elements include lanthanides with atomic numbers of 57˜71 and yttrium with an atomic number of 39, one or more of which are commonly selected to be used as additives for use in metallurgy, glass, chemical industry, nuclear industry, electronics industry, agriculture and pharmaceuticals as well as various functional materials due to their unique physical and chemical properties. A solvent extraction and separation method is not only a main method for separating and purifying a rare-earth element in industrial production of rare-earths at home and abroad, but also a main method for separating and preparing high-purity rare-earth compounds, which has advantages of high processing capacity, fast reaction rate and good separation effect. In recent years, efficient cleanness and separation of a rare-earth element has been becoming a research direction for rare-earths separation, and in 2011, “Emission Standards of Pollutants from Rare Earths Industry” introduced in China requires greener and more efficient rare-earth separation.
Recently, the separation process of a single rare-earth element is mainly achieved by a group separation of rare-earth elements with P507 (2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester) as an extractant (L I Deqian et al., phosphonate liquid-liquid extracting and separating rear-earth element, 1985, CN85102244). However, P507 has relatively small separation factor for heavy rare-earth elements, and meanwhile, leads to problems of high stripping acidity, incomplete stripping and long extraction equilibrium time. An acidic phosphine extractant, Cyanex272 (di(2,4,4-trimethylpentyl)phosphinic acid), when used for separating rare-earth elements, may improve the separation factor, however, the extractant Cyanex272 system is expensive, has a huge reagent and equipment investment, high quality requirements for feed liquid, and the separation system is easy to emulsify, and also its production process is difficult to control. L I A O Chunsheng et al. (Process for extracting and separating high purity lutetium oxide, 1998 CN1192479A.) used saponified P507 as an extractant with the addition of 0.5˜20% amine extractant N263 in the hydrochloric acid medium, to continuously separate and produce high-purity lutetium oxide from concentrates of thulium, ytterbium and lutetium or crude lutetium oxides through a one-step process. This system had overcome defects of P507 e.g. long extraction equilibrium time, difficult stripping and others; however, the composition of the extractant continually changed with the reaction, making it more difficult to control, and the acidity of the feed liquid when extraction with this system is 0.1 mol/L.
Under low acidity conditions, processes of extracting rare-earth ions with acidic organic phosphonic acid such as P507, di-(2-ethylhexyl)phosphoric acid (P204), etc., or organic carboxylic acid such as naphthenic acid, CA-12, etc., utilize a cation exchange mechanism, where a saponification (ammonia soap, sodium soap, magnesium or rare-earth soap and others) process is needed to enhance the extraction of rare-earth ions, and in addition to the introduction of corresponding ammonium or sodium, magnesium and other metal ions which leads to corresponding ammonia nitrogen or salinization pollutions, there is a need to consume a high-concentration acid and base to complete a series processes of extraction, washing and stripping. Moreover, P204 and others tend to generate emulsification at low acidity of the feed liquid, have much difficulty in the stripping of middle-heavy rare-earth element, and have much spent acid in strip liquor and large acid and base consumption (Chinese Patent ZL85102210). Even though under high acidity conditions (>2 mol/L), P507 and P204 may extract rare-earth ions using the P═O bonds in their molecules under non-saponified conditions, there may exist problems of high acid and base consumption and low extraction rate.
In addition, Chinese Patent CN100352954C discloses a process for extracting and separating a rare-earth element with addition of a modifier, which separates a single rare-earth element using a system of P507 added with alcohol, due to the difference among solubility of each component in the mixed system, long-time running will lead to changing of the extractant composition, which will affect the extraction effect of the system. The inventor considers providing a method for extracting and separating a rare-earth element, which uses an extractant which is not necessarily to be saponified and provides increased separation factor for the rare-earth element.